1. Field of the Invention:
This invention relates generally to a heterojunction bipolar transistor, and more particularly, to a heterojunction bipolar transistor incorporating separate dual base contacts for producing desirable current crowding.
2. Background of the Related Art
Transistors which are formed, at least in part, by one or more p-n junctions, in which the p-type and n-type semiconductor regions are of the same material, are generally referred to as homojunctions. Transistors which are formed of at least one p-n junction from semiconductor regions of dissimilar materials are generally referred to as heterojunctions. Because of the discontinuity of the energy band gaps of the two dissimilar materials, a heterojunction transistor exhibits a number of characteristic properties not found in their homojunction counterpart. Specifically, higher doping of the base region can be realized in a heterojunction transistor, thus reducing base resistance. Because of this, heterojunction transistors typically have use in a number of different applications.
A bipolar transistor typically is formed of two closely configured p-n junctions. For a heterojunction bipolar transistor (HBT) either of the materials forming the junctions can be dissimilar, however, it is the more conventional practice to have at least the emitter region and base region to be of the dissimilar materials. For bipolar transistors of this type, the conventional device generally includes a single conductive emitter contact, a single conductive collector contact and dual conductive base contacts formed on opposite sides of the emitter contact. Although there are usually two base contacts surrounding the emitter contact in the conventional HBT, the base contacts are electrically connected on the surface of the semiconductor block forming the bipolar transistor and are in essence a single contact.
The phenomenon known as current crowding in a transistor is conceptually defined as a higher current density near the edges of the base region of the transistor caused by a lateral voltage drop in the base region due to the base resistance. Because the current is forced into a particular area of the base region, the effective area of the active transistor region is generally reduced in an undesirable fashion. Consequently, certain design constraints on the transistor known in the art are implemented to reduce current crowding in the conventional bipolar transistor. Therefore, the effects of current crowding generally are not a major concern in the operation of a conventional HBT.
A reference disclosing a dual base homojunction bipolar transistor is acknowledged in the prior art. Specifically, in the article Wallace, "A Junction Transistor Tetrad For High-Frequency Use", Proceedings of the IRE, Vol. 49, November 1952, the authors disclose a separate dual base homojunction bipolar transistor in which the base resistance of the transistor is reduced due to the effects of current crowding. The object of this design was to reduce the path length of the current in the device, and thus, enable the transistor to operate at much higher frequencies.
As mentioned above, base resistance generally causes problems in effective transistor use. Additionally, as mentioned, base resistance limits the maximum frequency of operation of an HBT. Defects within the semiconductor material, whether they be surface defects or bulk defects, decrease the current gain of the transistor. Consequently, a method of determining where the defects exist at the development stage as a quality control step would increase the reliability of operation of the transistor in the marketed product.
Prior art methods of determining the location of defects in an HBT exist typically require testing of groups of different sized devices at the end of the manufacturing stage. By testing a group of simultaneously grown semiconductor devices, it is possible to determine the current characteristics of the entire batch. Since the accuracy of this type of test is generally effected by semiconductor devices having widely varying areas, it is necessary to test a group of the devices to localize defects. Consequently, the method of testing is inefficient in that a large number of tests must be performed to determine the location of the defects in a group of devices. Further, there is no way, from electrical measurement of a single device alone, to determine if defects in transistors are a surface or bulk phenomenon.
What is needed then is a method of determining defects in a semiconductor device without requiring a wide range of test devices. It is one object of this invention to utilize the phenomena of current crowding in an HBT to localize defects in the HBT. It is stressed, however, that inducing current crowding in an HBT has other applications as will become apparent.